I think that many are overlooking the primary reason that being able to sacrifice HP for DPS has very little chance of being a thing in PvE, that of the concept of autonomy vs dependency in terms of DPS capabilities for a job.
All jobs primarily rely on themselves and their own internal job mechanics to determine the DPS ceiling they can achieve, often relying on managing a resource that fuels DPS abilities. These resources are all self-contained within the job and are gained and used in a measurable and predictable fashion. HP is not that in any way and most HP that a tank has is managed and provided by another player, the healer.
Allowing DRK to use HP as a direct resource for DPS abilities will inevitably shift the responsibility for the DPS output of the DRK to their healer, turning the healer into a heal-bot resource battery to fuel the DRK's DPS. Nobody should want that kind of dynamic to be made a thing in the game.